1. Field of the Invention
The present invention relates to a driving device for driving a two-phase PM type stepping motor, and a lens driving device provided with the stepping motor driven by the driving device.
2. Related Background Art
A lens barrel device for moving a lens of a camera along an optical axis using a stepping motor driven with pulse signals is proposed, for example, in U.S. Pat. No. 5,384,506. In this lens barrel, the stepping-motor driving frequency and the number of driving pulses (number of steps) can be so controlled that the position of the lens with respect to the optical axis will be accurately adjusted compared to that using a DC motor or the like.
Though the above-mentioned patent teaches such a case that the lens is moved along the optical axis, the lens might be moved along a direction vertical to the optical axis in other devices such as a pickup device for reading out information recorded on an information recording disk such as a CD or DVD.
As another technique for precise control or adjustment of a position, there is a micro-step driving system in which current to energize coils is varied in stepwise fashion to stop a rotor of a motor in position according to the varied energizing current. FIG. 13 illustrates a method of energization in the micro-step driving system.
The micro-step driving system produces lower motor""s rotational speed and torque than a full-step driving system. This means that the micro-step driving system necessarily involves higher motor""s rotation resolution than the full-step driving system. Taking both advantages, an object can be placed in position using both driving systems. In other word, the full-step driving system is used for high-speed driving of the object when the object is far from a target location, and the micro-step driving system is used for precise positioning of the object when the object comes in close proximity to the target location. Thus high-speed, precise positioning can be achieved. The micro-step driving system is also used to gradually speed up the object during a low speed period for smooth startup, and the full-step driving system is used for high-speed driving of the object when the driving speed reaches a predetermined speed. This makes possible smooth, high-speed positioning of the driven object.
Upon stopping the rotor by micro-step control, however, the driving force of the motor""s rotation for positioning the object at the desired target location is reduced as the object is approaching the desired target location. In this case, even a little frictional force might make it difficult to place the object in the desired target location.
In contrast, if the rotor is stopped by full-step control, the driving force of the motor""s rotation becomes larger than the frictional force, which makes it easy to position the object. The reason why the full-step control makes it easy to position the object will be described below. To drive the rotor to the next rotational position, a driving circuit sends the motor a signal according to the next rotational position. The rotor, however, starts rotating with a slight delay after the driving circuit sends the signal. Thus a difference between the target rotational position and the actual rotor position occurs. If the difference is within a certain range, the driving force of the rotor""s rotation increases. Since the full-step driving can produce a larger amount of one-step rotation than that of the micro-step driving, the difference can also increase, which in turn makes the driving force larger. On the other hand, the micro-step driving produces a smaller amount of one-step rotation, and therefore, the difference cannot increase, which makes the driving force smaller than that of the full-step driving.
The motor""s driving force has only to increase in order to make positioning of an object easy. An increase in the motor""s driving force, however, needs an increase in current passed through coils, which may make electric power consumption high, or runs the danger of overheating the motor and hence lowering the motor""s characteristics. From this standpoint, it still has room for an improved configuration, which can achieve precise positioning of the rotor without passing excess current through the coils in vain.
On the other hand, U.S. Pat. No. 5,831,356 discloses a method of energization for switching from the micro-step driving mode to the full-step driving mode, and vice versa. FIG. 14 illustrates the energization method. Description will be made below by taking as an example switching from the full-step driving mode to the micro-step driving mode by varying energization of the coils in the motor. FIG. 15 illustrates the motor for use in switching from the full-step driving mode to the micro-step driving mode by the energization method shown in FIG. 14. FIGS. 16A and 17A are sectional views taken by Axe2x80x94A line in FIG. 15. FIGS. 16B and 17B are sectional views taken by Bxe2x80x94B line in FIG. 15. The rotational position of the rotor at T1 in FIG. 14 is shown in FIGS. 16A and 16B, and that at T2 in FIG. 14 is shown in FIGS. 17A and 17B. It is apparent from FIGS. 16A, 16B, 17A and 17B that the rotational positions of the rotor between T1 and T2 are displaced "THgr" degree with respect to each other. In other words, switching from the full-step driving mode to the micro-step driving mode causes a displacement in the rotor""s rotational position. Thus the energization method still leaves some to be desired to prevent a displacement in the rotor""s rotational position at the time of switching between driving modes.
Further, when the rotor of the motor is stopped and retained in the micro-step driving mode, the accuracy of stopping the rotor is susceptible to backlash of gear members such as gears and screws for transmitting rotations to the driven object, or inertial friction produced on a sliding portion. Therefore, it also has room for an improved configuration, which produces little or no ill effects mentioned above.
According to one aspect of the present invention, two different micro-step driving tables, each of which contains data on energizing amount for each step, are stored in a storage circuit. In case of driving the motor up to a number of steps set by an instruction circuit, a driving circuit energizes the motor on the basis of the data of the first micro-step driving table, and at a predetermined number of final steps, it energizes the motor on the basis of the data of the second micro-step driving table the data of which are set larger than those of the first micro-step driving table. In this configuration, even such a micro-step driving mode that it produces a small driving force just before the rotor is stopped can increase the driving force just before the rotor is stopped, which can achieve high-precision positioning against the frictional force.